In human anatomy, the spine is a generally flexible column that can take tensile and compressive loads. The spine also allows bending motion and provides a place of attachment for keels, muscles, and ligaments. Generally, the spine is divided into four sections: the cervical spine, the thoracic or dorsal spine, the lumbar spine, and the pelvic spine. The pelvic spine generally includes the sacrum and the coccyx. The sections of the spine are made up of individual bones called vertebrae. Also, the vertebrae are separated by intervertebral discs, which are situated between adjacent vertebrae.
The intervertebral discs function as shock absorbers and as joints. Further, the intervertebral discs can absorb the compressive and tensile loads to which the spinal column can be subjected. At the same time, the intervertebral discs can allow adjacent vertebral bodies to move relative to each other, particularly during bending or flexure of the spine. Thus, the intervertebral discs are under constant muscular and gravitational pressure and generally, the intervertebral discs are the first parts of the lumbar spine to show signs of deterioration.
In particular, deterioration can be manifested as a herniated disc. Weakness in an annulus fibrosis can result in a bulging of the nucleus pulposus or a herniation of the nucleus pulposus through the annulus fibrosis. Ultimately, weakness of the annulus fibrosis can result in a tear permitting the nucleus pulposus to leak from the intervertebral space. Loss of the nucleus pulposus or a bulging of the nucleus pulposus can lead to pinching of nerves and contact between osteal surfaces, causing pain and damage to vertebrae. In addition, aging can lead to a reduction in the hydration of the nucleus pulposus. Such a loss in hydration can also permit contact between osteal surfaces and pinching of nerves.
While replacement of the disc in the intervertebral space with an implant is an option, many surgeons seek a less invasive procedure. One such procedure for alleviating a bulging disc is injection of chemonucleolytic agents to dissolve a portion of the nucleus pulposus, drawing the herniated or bulging portion of the nucleus pulposus back to the center of the intervertebral disc. However, such injections can leak into sensitive regions outside of the intervertebral disc, leading to medical complications. Other attempts to alleviate bulging discs include implanting slowly dissolving solid matrices that include a chemonucleolytic agent. Once implanted, the solid matrices slowly dissolve, substantially releasing the chemonucleolytic agent, sometimes resulting in degradation of an excess amount of the nucleus pulposus.